TLC and HPTLC
Definition
Thin-layer and high-performance thin-layer chromatography. Silica gel plates eluted with ethyl acetate / ethanol / water or methanol / ethanol mixes; Rf values and band counts compare ink dye mixtures.
- Stationary phase
- Silica gel plates
- Common eluents
- Ethyl acetate/ethanol/water or methanol/ethanol mixes
- Measured output
- Rf values and dye band counts
- Compares
- Ink dye mixtures between questioned and known samples
Common questions
What is the difference between TLC and HPTLC in ink examination?+
HPTLC uses finer, more uniform silica particles and thinner layers than standard TLC, giving sharper band separation and better resolution of closely related dyes. Both run on the same principle, but HPTLC is preferred when dye mixtures are complex or similar.
How is an Rf value used to compare two inks?+
The Rf value is the distance a dye component travels relative to the solvent front, and it is reproducible for a given dye and solvent system. Matching Rf values and band colours between a questioned and a known ink support a common source, while a mismatch in either can exclude it.
Can TLC alone date a document?+
TLC identifies dye composition, which can show that an ink was not yet manufactured at a claimed document date, but it does not directly measure ink age. Dating typically needs additional solvent-loss or aging studies alongside the dye comparison.
Related terms
- Aginsky Method
- Solvent-extraction ink-age dating (Valery Aginsky, 1996). Compares fast and slow solvent extractions of phenoxyethanol to derive an aging parameter for ink under...
- ASTM E2789-11
- ASTM standard guide for forensic examination of inks (ink-age dating context). Pairs with ASTM E1422 for paper examination by FTIR.
- Ballpoint Ink
- An oil-based, high-viscosity ink using a resin binder (alkyd or polyamide) in a high-boiling solvent (phenoxyethanol or benzyl alcohol), with triphenylmethane dyes...
- Brunelle R-Ratio Method
- Dye-degradation method (Richard Brunelle and Kenneth Pro, 1972, later refined). Tracks the ratio of extracted and unextracted dye over time to estimate...
- Dye-Based Ink
- Soluble colourant dissolved in solvent. Modern fountain-pen and dye-based ballpoint inks belong here. TLC separates the dye mixture into resolvable spots.
- Gel Pen Ink
- Water-based gel vehicle with pigment particles (often copper phthalocyanine, iron oxide). Behaves like a pigment ink on TLC but more porous on...
- Iron-Gall Ink
- A historical ink formulation of ferrous sulphate, tannic acid (from oak galls), and gum arabic. The ferrous-tannate complex oxidises to brown ferric-tannate...
- LDI-MS / MALDI-TOF
- Laser-desorption mass spectrometry. Reads dye molecular ions directly from the ink stroke with minimal sampling, increasingly used for ink-library searching.
- Optical Brightener
- Stilbene-based fluorescent whitening agent added to paper from the early 1950s onwards; under 365 nm UV the paper glows bluish white. A...
- Phenoxyethanol Decay
- Loss of phenoxyethanol solvent from a ballpoint stroke over weeks to months, measurable by GC-MS or GC-FID. The basis of the Aginsky...
- Pigment Ink
- Insoluble colourant (carbon black, titanium dioxide, organic pigments) suspended in a binder. Pigment particles do not migrate on TLC, so Raman, FTIR...
- Raman Spectroscopy
- A non-destructive laser-based technique sensitive to molecular vibration modes in pigments; complements FTIR by characterising pigment molecular structure rather than bulk ink...